The present invention relates generally to analog to digital converters (ADCs) and more particularly to testing single-ended and differential successive approximation register (SAR) ADCs.
SAR ADCs are used to convert a continuous analog signal into a binary value. For example, a 12-bit SAR ADC converts an analog input voltage into a 12-bit binary representation. The SAR ADC operates using a binary search principle and converges to a digital output for each analog to digital conversion. A SAR ADC includes a digital-to-analog converter (DAC) and a comparator in a feedback loop with a SAR register. The DAC is formed of an array of binary weighted elements such as capacitors and/or resistors. Each binary weighted element corresponds to a binary digit of a corresponding binary representation. To generate a 12-bit binary representation, the DAC includes twelve (12) binary weighted elements and a dummy element.
During an analog to digital conversion, an input voltage is sampled during a sample phase. During a compare phase the DAC binary weighted elements are used to successively approximate the input voltage. At each successive approximation step, the comparator output is stored in the SAR register and the resulting digital word provides the digital representation of the analog input voltage.
Similar to other electronic components, ADCs are also prone to manufacturing defects, which include incorrect capacitance values, non-functional voltage switching circuit, defective comparator, and so forth. These defects lead to erroneous outputs from the ADCs. Thus, testing the ADCs for the above mentioned faults prior to being dispatched to customers is required.
Traditionally the ADCs are tested by providing test input voltages representing each of the conversion result values. To account for noise and to precisely calculate the error, the test input voltage is varied in multiple steps in the range of each conversion result value. However, this test technique is time consuming, which leads to prolonged test times and requires expensive test equipment, both of which directly increase test costs.